INTRODUCTION 7 



depending entirely on the composition, temperature, 

 density, humidity, etc., of the nutrient medium. This 

 discovery has been corroborated and extended by other 

 observers. 



Its significance to the systematist, whose specific and 

 even generic characters in certain groups of fungi often 

 depend on the union or separation of the sexes, relative 

 abundance of antheridia, etc., is obvious. 



Klebs also considers that there is no inherent law 

 demanding an alternation of generations. Under ordinary 

 circumstances in nature, this is effected mainly through 

 the exhaustion of food in each successive host-plant 

 attacked. 



This theory appears to account for the alternation of 

 generations, even in such obligate parasites as the 

 Uredineae. When the aecidiospores of Pucdnia graminis 

 pass from barberry to wheat, the food in the latter host- 

 plant is not conducive to the formation of aecidiospores, 

 and uredospores are produced. As the wheat plant 

 matures, the composition of the food available for the 

 parasite changes; as a consequence uredospores are no 

 longer formed, but are gradually replaced by teleutospores. 



In Australia, Pucdnia graminis is rampant, but the host- 

 plant of the aecidium stage is absent, consequently the 

 aecidium has dropped out of the cycle of development of 

 the fungus, which, however, still produces uredospores 

 and teleutospores on the one host necessary for this 

 purpose. 



Other instances are known where one of the factors in 

 the life-cycle has disappeared without in any way inter- 

 fering with the continuance of the species. 



On the other hand, alternation is sometimes suspended, 



